Apparatus and method for charging and discharging a battery

ABSTRACT

A method and apparatus for controlling the charge and discharge currents in a battery ( 2 ) as a function of temperature. When a battery ( 2 ) is charged or discharged in an environment that approaches its design operating temperature extreme, the currents are reduced to limit self-heating of the battery and thus extend the useful operating environment temperature range. A temperature sensor ( 18 ) is coupled to a controller ( 6 ) to sense the battery ( 2 ) temperature. The temperature information is used to set a suitable charging or discharging current ( 8 ). In the illustrative embodiment, the charging current is set to a maximum value when said temperature is lower than a first predetermined threshold value, the maximum value being the battery&#39;s maximum specified charging current, and the first predetermined threshold value being the battery&#39;s maximum charging temperature.

CROSS-REFERENCE TO RELATED APPLICATION

This is a Continuation of U.S. patent application Ser. No. 10/011,140filed Nov. 12, 2001 now U.S. Pat. No. 6,661,203 by D. Wolin et al. andentitled BATTERY CHARGING AND DISCHARGING SYSTEM OPTIMIZED FOR HIGHTEMPERATURE ENVIRONMENTS.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to apparatus and methods for chargingrechargeable batteries. More specifically, the present invention relatesto apparatus and method for charging, discharging and rechargingrechargeable batteries under adverse thermal conditions.

2. Description of the Related Art

Reliable electric power sources are needed to meet the continued growthof electric and electronic business, commercial and personalapplications. For portable applications, the chemical storage battery ismost commonly employed. For fixed location applications, the publicpower grid is the most common source of electrical power. Also,alternative sources of power are often employed to produce electricpower, such as solar-voltaic, thermal, wind, water and other powersources.

For many applications, a high degree of reliability is required.Although public power grids are highly reliable, these grids are notperfect. Nor are alternative sources of electric power. Therefore,storage batteries are frequently employed in conjunction with, and as aback-up to, the public power grid and alternative sources of electricalpower.

Chemical storage batteries have been produced using a variety oftechnologies. Each technology comprises a number of definingcharacteristics that should be considered in selecting a suitabletechnology for a particular application. These include, but are notlimited to, size, weight, cost, power density, environmentalconstraints, voltage, current, power, and so forth.

In many applications, the ability to be recharged is a criticalrequirement of a chemical storage battery. Rechargeability reduces cost,extends useful life, and adds reliability to both battery and systemdesign. Some common chemical technologies employed in rechargeablebatteries are Nickel-Metal Hydride, Lithium Ion, Lithium Ion Polymer,Lead-Acid, and Nickel Cadmium among other unique and hybridtechnologies.

Rechargeable batteries are charged by delivering electric current topositive and negative terminals of the battery for a duration of timesufficient to fully charge the battery. Later, current is drawn from thebattery as a power source to some particular device or application.

However, the conditions of charging and discharging are not withoutlimitations. The limitations are typically defined by the batterymanufacturer or supplier. In applications where a battery is maintainedas a back up to another primary source of electrical power, the batterymay rest for long periods of time in a fully charged (“standby”) state,awaiting an interruption of the primary power source. When this occurs,the electric power stored in the battery is consumed in lieu of theprimary power source.

A chemical battery resting in the standby state for long periods of timemay degrade due to various factors. The total power available may bereduced, the terminal voltage may change, and the ability to determinethe amount of power available may be compromised.

Smart battery charge algorithms have been developed to alleviate some ofthe problems associated with long term standby operation of a battery.Such chargers periodically ‘condition’ the battery by applying anartificial load to discharge the battery to some predetermined level,and then recharge the battery to full charge. During such a conditioningprocess, certain metrics may be measured and used to calibrate thebattery for later determination of the available power during a batterydischarge cycle. It is desirable to process a discharge cycle in asshort a period of time as possible so that the battery can quickly bereturned to standby operation. Similarly, it is generally desirable tocharge a battery as quickly as possible so that it can be readied foruse as quickly as possible.

When a battery is being charged or discharged, a certain amount ofinternal heat is generated as current flows through the battery. Thisheat is proportional to the amount of current flowing within thebattery. In ambient conditions where the amount of heat generated issmall compared to the heat loss from the battery, the internal heatgeneration is usually not significant. Often, a battery is located inclose physical proximity to the device it powers or to which it providesstandby service. An example of this is occurs when a battery is used toprovide standby power to a computing device. In most instances, thedevice with which the battery operates also generates heat duringoperation.

Electrical energy discharged from the battery can cause thermal problemsat high temperature, for both the battery and the adjacent circuitry.For example, a battery may be subjected to heat energy produced by thedevice it powers as well as the heat the battery produces internally. Inaddition, the components adjacent to the battery conditioning circuit(often a resistive load) may be pushed close to thermal limits due tojoule heating of the discharge load at high temperature.

In addition, other heat sources in the vicinity of the battery mayaffect ambient conditions and raise the operating temperature of theenvironment. Thus, it is not uncommon for a battery to be operated atsubstantially elevated temperatures.

When a battery is operating at or near its maximum operatingtemperature, designers are faced with a dilemma. If the battery chargeand discharge currents are maintained at levels normally applied for thelower ranges of expected operating temperatures, the battery life andreliability can be greatly compromised when temperatures becomeelevated. On the other hand, if the designer takes a conservativeapproach, and sets the charge and discharge currents at levelsconsistent with a reasonable maximum operating temperature, then chargeand discharge currents may be so low that the time required toaccomplish these operations become unacceptably long.

Alternatives presently available to address this dilemma includelocating the battery in a cooler environment, usually distant from thedevice being powered and providing additional cooling equipment. Each ofthese alternatives is typically undesirable due to increased cost,greater systems complexity, or reduced reliability, inter alia.

Thus there is a need in the art for an apparatus and method forefficiently charging, discharging and recharging batteries inenvironments with variable thermal conditions.

SUMMARY OF THE INVENTION

The need in the art is addressed by the apparatus and methods taught bythe present invention. An apparatus for charging a battery according toits temperature is taught. The apparatus includes a charging circuitadapted to charge a battery and a temperature sensor positioned to sensea battery temperature, i.e., adjacent environmental temperature. Theapparatus includes a controller coupled to the temperature sensor andthe charging circuit. The controller operates to set the chargingcurrent in accordance with the sensed temperature. The charging currentis set to a maximum value when the temperature is lower than a firstpredetermined threshold value, the maximum value being the battery'smaximum specified charging current, and the first predeterminedthreshold value being the battery's maximum charging temperature.

In a refinement, the controller continuously sets the charging currentin accordance with the sensed temperature. In a further refinement, thecontroller periodically sets the charging current in accordance with thesensed temperature. In a further refinement, the apparatus furtherincludes a memory coupled to the controller having a temperature andcharging current look up table stored therein. In this embodiment, thecontroller accesses the look up table to set the charging current. In afurther refinement, the controller operates to set the charging currentto a maximum value when the temperature is lower than a firstpredetermined threshold value. In a further refinement, the maximumvalue is the battery's maximum specified charging current and the firstpredetermined threshold value is the battery's maximum chargingtemperature. In a further refinement, the controller sets the chargingcurrent to zero when the temperature is higher than a secondpredetermined threshold value. In a further refinement, the battery iscoupled to a load and the temperature sensor senses the temperature ofthe battery and the load.

The present invention also teaches an apparatus for exercising orconditioning a battery. This apparatus includes the charging circuit anda temperature sensor. Also, a discharging circuit is coupled to thebattery while a controller is coupled to the temperature sensor, thecharging circuit, and the discharging circuit. The controller operatesto set the charging and discharging currents in accordance withtemperature.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a functional block diagram of an illustrative embodiment ofthe present invention.

FIG. 2 is a flow diagram of an illustrative embodiment of the presentinvention.

FIG. 3 is a flow diagram of an illustrative embodiment of the presentinvention.

DESCRIPTION OF THE INVENTION

Illustrative embodiments and exemplary applications will now bedescribed with reference to the accompanying drawings to disclose theadvantageous teachings of the present invention.

While the present invention is described herein with reference toillustrative embodiments for particular applications, it should beunderstood that the invention is not limited thereto. Those havingordinary skill in the art and access to the teachings provided hereinwill recognize additional modifications, applications, and embodimentswithin the scope thereof and additional fields in which the presentinvention would be of significant utility.

The present invention advantageously utilizes a temperature sensor incombination with a battery charger, or a battery conditioner, to controlcharging and discharging current flow as a function of the batterytemperature. As is understood by those skilled in the art, rechargeablebatteries are characterized by a number of operational constraints.Among these are terminal and charging voltage, maximum charging currentflow, maximum current draw, and a range of environmental constraints,including maximum operation, charging and discharging temperatures.Implementation of a battery in a system that operates outside the boundsof such constraints will lead to a number of deleterious effects. Theseinclude reduced battery life, reduced battery capacity, and certainpotentials for dangerous situations including overheating, fire, andchemical leakage. Thus, designers strive to maintain operational factorswithin design constraints. Yet, in certain practical applications,designers are forced to implement batteries in environments that pushthe limits of these constraints. The present invention allows designersto move closer to the absolute limits, while still maintaining maximumperformance from the battery and the system into which it is installed.

As discussed above, a number of chemical technologies are employed inmodern rechargeable batteries. Each technology is constrained as notedabove. In an illustrative embodiment, a lithium ion battery is employed.Nonetheless, it will be understood by those of ordinary skill in the artthat the teachings provided herein are not limited to a particularbattery technology.

When a battery is charged or discharged, current flows through thebattery and a certain amount of internal heat is produced. When thebattery is being charged or discharged in an environment near itsmaximum operating temperature, the internal heat generated can push thebattery beyond it design constraints, leading to the aforementioneddeleterious effects. Operating environments that are near a battery'soperating extremes are rather common. For example, a battery back-upsystem for a computing device, such as a computer or mass storagesystem, is often times located in close proximity to the computingdevice. The heat produced by the computing device contributes to theheat of the environment that the battery operates within. Also ambientconditions may be warm or hot, exacerbating the thermal environment.There are many other applications that push the thermal constraints,including outdoor, mobile, industrial, non-air conditioned, and othersimilar environments. The present invention advantageously balances thecurrent flow in the battery, thus balancing the internal heat generationand build-up, with the battery and local environmental temperature.

In an illustrative embodiment, a lithium ion smart battery is employedin a computer storage disk array system and the present invention isimplemented to allow the system to extract maximum performance from thebattery without exceeding safe operational constraints for the battery.

The Smart Battery industry standard describes one or more battery cellsin conjunction with a controlling device that enables the battery tomeasure and communicate certain information about its operation to auser or an external device. An implementation of a Smart Battery, whichis the battery employed in an illustrative embodiment of the presentinvention, is the Moltech Power Systems model NI2040A17 RechargeableLithium Ion Battery, specifications for which are available from MoltechPower Systems, Inc., 12801 NW Highway 441, Alachua, Fla. 32615. ThisSmart Battery employs lithium ion chemistry in nine storage cells thatare arranged in a three by three series-parallel configuration to yielda nominal terminal voltage of 10.8 volts and a power rating of 5000milli-Ampere hours (“mAhr”). The Smart Battery employs a controller anda “fuel gauge” which is coupled to a display that indicates thebattery's power reserve in twenty percent increments. The Smart Batterycomprises a thermistor temperature sensor within its housing. The SmartBattery also comprises an SMBus two-wire serial communications port, asis understood by those possessing ordinary skill in the art. The SMBusinterface generally applies the industry standard I²C signaling levels.The SMbus is operable to communicate the smart battery's terminalvoltage, the rate of current flow into or out of the battery, thecharges state, including whether the battery is fully charged or fullydischarged, and the temperature of the battery, according to theaforementioned thermistor temperature sensor. In the illustrativeembodiment, the SMBus is coupled to a host controller, as will be morefully discussed herein after. The illustrative embodiment Smart Batteryspecifications provide a maximum charge current to 3 amperes at 12.6volts in the range of temperatures from 0° C. to 45° C. Discharge israted at 3 amperes from 0° C. to 50° C. Full charge is realized when thecharging current drops below 150 milli-amperes.

Reference is directed to FIG. 1, which is a functional block diagram ofan illustrative embodiment of the present invention. The aforementionedSmart Battery 2 comprises a plurality of lithium ion cells 24 that arearranged in a series-parallel configuration. A thermistor 18 is locatedwithin the battery 2 at a position enabling it to sense the temperatureimmediately adjacent to the battery cells 24. The thermistor 18 iscoupled to a controller 16, which is operable to read the temperaturevia the thermistor 18. The controller 16 is coupled to a current sensor20 that enables the controller 16 to monitor the current flow throughthe battery 2. The controller 16 is also coupled to a voltage sensor 22that enables the controller 16 to monitor the battery 2 terminalvoltage. A fuel gauge 26 is provided that displays the remaining batterycapacity, as well as making this information available to the controller16.

The battery 2 controller 16 is coupled to an SMBus 4 enablingcommunications of the aforementioned parameters through the SMBus 4. Thepositive and negative output terminals of battery 2 are coupled thoughrelay 12 to a load, which is a computing device 10 in this illustrativeembodiment. A programmable charger 8 is coupled to the battery 2 andenables the supply of charging current to the battery 2. The charger 8comprises an SMBus interface coupled to SMBus 4, which interface allowsthe charger 8 to be programmed to deliver a specified current andvoltage to the battery 2 for charging thereof. A host controller 6 iscoupled to the SMBus 4 and is operable to control the operation of thisembodiment of the present invention. The host controller 6 is alsocoupled 14 to actuate relay 12, which may be accomplished eitherdirectly (as shown) or through an SMBus interface (not shown). The hostcontroller 6 may be any of a variety of processors, microprocessors,controllers, microcontrollers, or other programmable devices as arepresently understood, or later become available, to those possessingordinary skill in the art. The host controller includes an amount ofrandom access memory in the illustrative embodiment. The temperaturesensor may be a thermistor, a thermocouple, an infrared sensor, or anyother sensor having an output proportional to temperature that isunderstood by those possessing ordinary skill in the art.

The host controller 6 memory is programmed with a look up table ofcharging and discharging currents related to temperatures. In theillustrative embodiment, these relations are determined throughempirical measurements. Table 1 below shows illustrative chargingcurrent and temperature values:

TABLE 1 Temperature Current Less than 45° C. 2.0 Amps 45° C. to 55° C.1.0 Amps 55° C. to 60° C. 0.5 Amps Greater then 60° C. 0.0 Amps

In operation, the host controller 16 periodically requests the batterytemperature from the smart battery 2 and uses this value to access thememory look up table to select a charging current associated with thattemperature. By applying these reduced current values, a correspondingreduction in the self-heating of the battery cells is caused by thecurrent flow. This reduction allows the battery to function in acorrespondingly warmer environment at the system level. For example, areduction of the charge current by 50% will reduce the power, and hencethe self-heating term, by the square of the charge, or 75%. This readilyprovides an improvement of 5° C. compared to the battery suppliersrecommend extreme temperatures of operation.

As may be expected with respect to the charging cycle, a self-heatingterm is associated with cell temperatures due to the discharging cycle.When a battery is conditioned, or exercised, the system discharges thebattery to a predetermined level. This allows the system to calibratethe battery and assess capacity and useful life, as is understood bythose skilled in the art. The battery is then recharged, as discussedabove. The discharge rate is reduced in like fashion to the charge rate,thus, reducing self-heating and extending the battery's useful life.Also note that the discharge current is directed to a load, such as aresistive load, that converts the battery energy into heat as it isdischarged. In the illustrative embodiment, a variable impedance load,under control of the host controller, is employed. A look up table inthe memory is used to recall empirically derived factors for suitabledischarge current rates, in like fashion with respect to the chargingapproach. The load is typically located in close proximity to thebattery and thus the heat produced affects the battery's environment.The temperature sensor should be positioned to detect this heat, therebyallowing the system to respond accordingly.

Reference is directed to FIG. 2, which is a flow diagram of anillustrative embodiment of a charging operation according to the presentinvention. The process is called by the host controller at step 30 andproceeds to read the battery temperature at step 32. The batterytemperature returned is used to access the look up table in the memoryat step 34. The current associated with that temperature is recalled andused to set the output current of the charger at step 36. At step 38,the host controller reads the charge state over the SMBus to determinewhether the battery is fully charged or not. If the battery is fullycharged at step 38, then the process returns to the calling routine atstep 40. On the other hand, if the battery is not fully charged at step38, then the flow returns to step 32 to repeat the process.

The foregoing describes an operation where the battery temperature iseffectively continuously tested by the reiterative loop. In a practicalapplication, it may be preferred to add a fixed time delay because thethermal mass of the battery will prevent sudden jumps in temperature.Thus, the process can readily be adapted from a continuous test to aperiodic test, suitable for a given application and environment.

Reference is directed to FIG. 3, which is a flow diagram of anillustrative embodiment of the conditioning, or exercise, operationtaught by the present invention. The process is called by the hostcontroller at step 50 and proceeds to read the battery temperature atstep 52. The battery temperature returned is used to access the look uptable in the memory at step 54. The discharge current associated withthat temperature is recalled and used to set the load impedance, ordischarge current at step 56. At step 58, the host controller reads thecharge state over the SMBus to determine whether the battery is fullydischarged or not. If the battery is fully discharged at step 58, thenthe process proceeds to step 60 where the charging process of FIG. 2 isexecuted. After the charging process is completed, flow returns to thecalling routine at step 62 in FIG. 3.

On the other hand, if the battery is not fully discharged at step 58 inFIG. 2, then the flow returns to step 52 to repeat the process. Theforegoing describes an operation where the battery temperature iseffectively continuously tested by the reiterative loop. In a practicalapplication, it may be preferred to add a fixed time delay because thethermal mass of the battery will prevent sudden jumps in temperature.Thus, the process can readily be adapted from a continuous test to aperiodic test, suitable for the application and environment at hand.

Thus, the present invention has been described herein with reference toa particular embodiment for a particular application. Those havingordinary skill in the art and access to the present teachings willrecognize additional modifications applications and embodiments withinthe scope thereof.

It is therefore intended by the appended claims to cover any and allsuch applications, modifications and embodiments within the scope of thepresent invention.

1. An apparatus for exercising a battery, comprising a charging circuithaving a charging current output coupled to the battery; a temperaturesensor positioned to sense a temperature related to the batterytemperature; a discharging circuit having a variable impedance load anda discharging current input coupled to the battery; a controller coupledto said temperature sensor, said charging circuit, and said dischargingcircuit, said controller operable to set said charging current inaccordance with said temperature, and operable to set said dischargingcurrent in accordance with said temperature, said controller beingoperable to minimize said charging current when said temperature ishigher than a first predetermined threshold value; and a memory coupledto said controller having a look up table with temperature versusdischarging current and values of said variable impedance load storedtherein, whereby said controller accesses said look up table to set saiddischarging current.
 2. The apparatus of claim 1 and wherein saidcontroller continuously sets said discharging current in accordance withsaid temperature.
 3. The apparatus of claim 1 and wherein saidcontroller periodically sets said discharging current in accordance withsaid temperature.
 4. The apparatus of claim 1 and wherein saidcontroller is operable to set said discharging current to a maximumvalue when said temperature is lower than a second predeterminedthreshold value.
 5. The apparatus of claim 1 wherein said maximum valueis the battery's maximum specified discharging current and said firstpredetermined threshold value is the battery's maximum dischargingtemperature.
 6. The apparatus of claim 1 wherein said temperature sensorsenses the temperature of the battery and said discharging circuit.
 7. Amethod of exercising a battery, comprising the steps of: sensing atemperature related to the battery temperature; setting a dischargingcurrent in accordance with said temperature by recalling a dischargingcurrent corresponding to said sensed temperature from a look up table;discharging the battery at said discharging current with a dischargingcircuit having a variable impedance load, the impedance of said loadbeing selected from said look up table; discontinuing said dischargingstep when a predetermined battery voltage is reached; setting a chargingcurrent in accordance with said temperature, said setting step furtherincluding the step of minimizing said charging current when saidtemperature is higher than a first predetermined threshold value; andcharging the battery at said charging current.
 8. The method of claim 7and wherein said sensing and setting a discharge current steps arerepeated continuously during said discharging step.
 9. The method ofclaim 7 and wherein said sensing and setting a discharge current stepsare repeated periodically during said discharging step.
 10. The methodof claim 7 and wherein said setting step includes setting saiddischarging current to a maximum value if said temperature is lower thana second predetermined threshold.
 11. The method of claim 10 and whereinsaid maximum value is the battery's maximum specified dischargingcurrent, and said first predetermined threshold is the battery's maximumdischarging temperature.
 12. The method of claim 7 wherein the batteryis coupled to a load, and wherein said sensing step includes the step ofsensing the temperature of the battery and the load.